Study of Effective Factor on Fatigue Strength of (α+γ) Dual Phase Stainless Steel by Shot Peening Process

A shot peening process is generally utilized as one of the surface treatment processes to improve the fatigue life and the fatigue strength of cyclically loaded components. This improvement is achieved by inducing compressive residual stresses and work hardening effects in areas close to the surface. But the influence of the shot peening process on fatigue strength is not clarified enough. It's significant technologically and industrially to clarify the influence factor of shot peening on the fatigue strength. Former study of author confirmed that main effect on the fatigue strength of the SUS304 steel hitted by controlled steel ball is the hardening of austenite structure itself due to the shot peening. In this study, the SUS329J1 steel having both austenite and ferrite phases was also investigated from the viewpoint of the work hardening of both structures. An effect of the residual stress on fatigue limit is not observed at all. As a result, we found that the work hardening of the SUS329J1 steel by shot peening was attributed to both the strain-induced martensitic transformation and the hardening of austenite and ferrite phase themselves. Especially, the ferrite phase influences the Vickers hardness distribution of shot-peened material, and the hardness increment of the austenite phase is larger than other phase and mainly due to the fine structure.